1. Field of the Invention
The present invention relates to a spread illuminating apparatus for use in a display portion of a personal computer, a word processor or the like (hereinafter, they are referred to as “a personal computer”), or a mobile apparatus, a portable telephone or the like.
2. Description of the Related Art
As for display devices for use in personal computers, heretofore, cathode ray tubes (so-called picture tubes) have been mainly used. However, since these cathode ray tubes are large and heavy and in addition thereto have large power consumption, recently, liquid crystal panels have been employed in many cases. Therefore, the liquid crystal panels each having a large number of pixels have been developed and other circuit elements have been miniaturized, leading to that small personal computers such as a note-book type or the like have rapidly come into wide use. The liquid crystal panels have been often employed as the display devices for use in the mobile equipments, the portable phones or other various electronic equipments in addition to the above-mentioned personal computers.
While the demand for the liquid crystal panels has drastically increased as described above, a suitable illuminating apparatus is required, because the liquid crystal panel itself does not emit the rays of light. Conventionally, in a display device having a relatively large area as in a personal computer, for example, the cold cathode fluorescent lamps (CCF) or the hot cathode fluorescent lamps (HCFL) have been used as the illuminating apparatuses therefor. As for the concrete construction thereof, a square transparent substrate is provided in close proximity to the surface of a liquid crystal panel, the transparent substrate is illuminated by a fluorescent tube which is provided parallel to the side surface of the transparent substrate, and then the rays of light which are emitted from the fluorescent tube are introduced to the liquid crystal panel which is in turn illuminated with the rays of light introduced thereto.
Since the discharge of electricity is caused between the opposite electrodes in the fluorescent tube, a high voltage for the discharge of electricity is required. For this reason, an oscillation circuit employing a semiconductor device is provided to supply the voltage which is generated on the basis of the oscillation to the fluorescent tube.
Because the personal computer is constructed by combining the electric circuits and the electric apparatuses, the personal computer itself consumes the electric power. Thus, the study for power saving has been made and as a result, currently, the personal computers with remarkably reduced power consumption as compared with the conventional ones have come into wide use. In particular, this tendency is remarkable for the portable personal computers called the note-book type. However, if the amount of power consumption of the illuminating apparatus is large, then the greater part of the electric power generated by the batteries will be consumed in the illuminating apparatus.
On account of this, an illuminating apparatus that employs a white light emitting diode instead of the fluorescent tube has been developed. The patent application of this illuminating apparatus has already been filed with the Patent Office by the present inventors (refer to Japanese Patent Laid-open Publication No. Hei 10-182076). This illuminating apparatus is structured such that a transparent substrate is placed on a liquid crystal panel, a bar-like light conductive member which is made of a transparent material and a part of which is provided with an optical path conversion means is arranged on the end of the transparent substrate, and a light emitting diode is placed on the end portion of the light conductive member in order to emit the rays of light. The rays of light which have been emitted from the light emitting diode are guided from the end portion of the light conductive member into the light conductive member and then the direction of the emitted rays of light is changed by the optical path conversion means so that the rays of light directly enter the transparent substrate.
Now, whether the bar-like light source is comprised of the fluorescent tube or the combination of the light conductive member and the light emitting diode, it is impossible to brightly illuminate the liquid crystal panel which is arranged in contact with the lower surface of the transparent substrate unless the transparent substrate is uniformly illuminated. Therefore, various ideas have been proposed so far.
Now, an example of a conventional spread illuminating apparatus will be hereinbelow described with reference to FIG. 8 which is an exploded view of the conventional spread illuminating apparatus and FIG. 9 which is a perspective view showing the assembled state thereof. A liquid crystal panel 1 is a reflection type liquid crystal device and has on its surface liquid crystal cells 1a as pixels disposed in a mosaic arrangement. A square transparent substrate 2 is provided close to the upper surface of the liquid crystal panel 1, and a fluorescent tube 3 as a bar-like light source is provided parallel with the side surface of the transparent substrate 2.
In this construction, when the fluorescent tube 3 is lit, the rays of light which have been emitted from the fluorescent tube 3 enter the transparent substrate 2 to illuminate the liquid crystal panel 1 through the transparent substrate 2. In order that the rays of light emitted from the fluorescent tube 3 may be effectively directed toward the transparent substrate 2, a reflection plate 4 is provided behind the fluorescent tube 3.
A large number of groove portions 2a each having a substantial triangle in cross section are provided on the surface of the transparent substrate 2 in such a way as to be parallel with the fluorescent tube 3. Each of the portions which are defined between the adjacent groove portions 2a forms a plane portion 2b. In order to realize the uniformity of the surface light emission, the gaps between the groove portions 2a are large in the vicinity of the fluorescent tube 3 and become smaller as the distance from the fluorescent tube 3 increases. In addition, the groove portions 2a are shallow in the vicinity of the fluorescent tube 3 and become deeper as the distance from the fluorescent tube 3 increases.
In this connection, when the fluorescent tube 3 emits the rays of light, the rays of light thus emitted therefrom enter the transparent substrate 2 at the side surface thereof and strike against the groove portions 2a to be directed toward the liquid crystal panel 1. However, only the components of the rays of light vertical to the groove portions 2a undergo optical path conversion. Therefore, in FIG. 9, only the components of the rays of light in the direction A are converged, while the components of the rays of light in the direction B parallel with the groove portions 2a are not converged at all. As a result, as shown in FIG. 7, the distribution in the direction B is widely spread. Note that, the rays of light which are directed from the transparent substrate 2 toward the liquid crystal panel 1 are reflected by the liquid crystal panel 1, thereby entering the transparent substrate 2 again to be emitted in the viewing direction.
When the illuminating apparatus for the liquid crystal display device is used in the portable information equipment such as a portable phone, a wide view angle is not required assuming that it is for personal use. The fact is that the rays of light emitted in a wide direction turn out to be useless. Therefore, the useless rays of light emitted in the direction B are converged, whereby it is possible to enhance the front viewing luminance. In order to converge the rays of light emitted in the direction B, additional groove portions need to be formed in the direction perpendicular to the direction of the formation of the existing groove portions. However if the groove portions are formed substantially in parallel with the travelling direction of the rays of light, it is still impossible to converge the rays of light.
If the transparent substrate 2 is provided on the upper surface of the liquid crystal panel 1 as in the above structure, then interference fringes called the Moiré fringes may be generated on the viewing face of the liquid crystal panel 1 in some cases. When the Moiré fringes are generated, the liquid crystal panel 1 becomes hard to view. It has been discovered that the generation of the Moiré fringes has a close relation with the depth, size and gap of the groove portion 2a of the transparent substrate 2. In addition thereto, it has also been known that the Moiré fringes are generated most intensely when the groove portions 2a are substantially parallel with the direction in which the liquid crystal cells 1a of the liquid crystal panel 1 are arranged.